Conventionally, there has been disclosed an exhaust gas NOx removal device comprising: supply means configured to supply a reducing agent for nitrogen oxides, and provided in a flow passage of a nitrogen oxide-containing exhaust gas; and an exhaust gas NOx removal unit provided at a downstream side of the supply means (see Patent Document 1, for example). In this exhaust gas NOx removal device, the supply means of the reducing agent includes: a urea solution ejecting unit for ejecting a urea solution; an evaporating unit for evaporating the urea solution ejected by the urea solution ejecting unit; a hydrolyzing unit provided at a downstream side of the evaporating unit in a manner to decompose the urea solution into an ammonia gas; and a reducing agent injecting unit for injectingly supplying the ammonia-containing gas into the exhaust gas. The device further comprises a heater(s) provided at the evaporating unit, or at the evaporating unit and the hydrolyzing unit. Further, the urea solution ejecting unit, evaporating unit, hydrolyzing unit, and reducing agent injecting unit (decomposed gas outlet portion) are provided in this order within a cylindrical vessel to thereby constitute a urea evaporator.
In the exhaust gas NOx removal device as constituted above, when the cylindrical vessel constituting the urea evaporator is uniformly heated in a manner to blow thereinto a urea solution (urea concentration between 30% and 50%), water and urea are caused to evaporate within the vessel, such that the urea is decomposed at surfaces of catalyst particles such as γ-alumina particles, potassium carbonate-carrying particles, or the like filled in the vessel, thereby producing an ammonia gas. Here, the evaporator has an interior structure configured to keep 450° C. or higher to thereby promote rapid evaporation by heating, so as to prevent production of solid by-products such as cyanuric acid, isocyanic acid, and the like to be otherwise produced in the process of thermal decomposition of urea. In other words, the structure of the evaporator is elaborated to promote heat transfer at an evaporating surface, in a manner to be capable of restricting a temperature fluctuation within the vessel even at a considerable fluctuation of a flow rate of injected urea. Specifically, the structure of the evaporator is filled, at an upper side (or upstream side) of γ-alumina particles or the like filled in the hydrolyzing unit, with silicon carbide, iron or stainless balls, metal honeycomb, or the like, which are better in thermal conductivity than the γ-alumina particles.
Further, there has been disclosed a device for producing a gas flow containing a reducing agent (hereinafter called “reducing agent-containing gas flow producing device”) comprising: a duct having a first zone for partially evaporating a reducing agent precursor to produce a gas flow, and a second zone for partially heating the gas flow, wherein the duct is provided with: transporting means for supplying the reducing agent precursor; means for converting the reducing agent precursor in the gas flow into a reducing agent; and heating elements for heating the first zone to a first temperature and heating the second zone to a second temperature, respectively (see Patent Document 2, for example).
In the reducing agent-containing gas flow producing device as constituted above, the transporting means supplies the reducing agent precursor to the duct, the heating element positioned in the first zone evaporates the reducing agent precursor to produce a gas flow, and thereafter the heating element positioned in the second zone partially heats the gas flow up to a temperature of 250° C. to partially convert the reducing agent precursor in the gas flow into a reducing agent, thereby producing a reducing agent-containing gas flow. The reducing agent-containing gas flow produced in the reducing agent-containing gas flow producing device is introduced into an exhaust line of an internal combustion engine, and is mixed there with an exhaust gas flow of the internal combustion engine. The device is configured so that the mixed gas of the reducing agent-containing gas flow and exhaust gas flow is caused to flow through an SCR catalyst converter such that nitrogen oxides contained in the exhaust gas flow are converted by the reducing agent, thereby decreasing a content ratio of NOx in the exhaust gas.
In turn, there has been disclosed a mixed gas supplying device including a reservoir of a water solution (urea water solution, for example) containing at least one kind of reducing agent precursor substance, wherein the reservoir is connected to an evaporating chamber, the water solution is supplied to the evaporating chamber by supplying means, and the mixed gas contains at least one substance comprising at least one kind of reducing agent or at least one kind of reducing agent precursor substance (see Patent Document 3, for example). This mixed gas supplying device further includes heating means arranged for the evaporating chamber and formed of a heating wire contacted with the evaporating chamber, such that the urea solution in the evaporating chamber is heated to a temperature at or above a critical temperature where the urea water solution is caused to be at least partially evaporated by the heating means. Specifically, an evaporating device has the evaporating chamber having a substantially closed volume, and the evaporating chamber has a first opening for connecting a delivery line for delivering a urea water solution, and a second opening for connecting a supply line for discharging the mixed gas therefrom. Arranged at the first opening is a nozzle as means for injectingly supplying a urea water solution into the evaporating chamber, so that the nozzle injects the urea water solution into the evaporating chamber. Further, the evaporating chamber includes, at the site of the second opening, means for preventing invasion of droplets into the second opening, particularly, means (such as a protrusion at a wall) for breaking a gas film situated between droplets and a wall of the evaporating chamber, and the evaporating chamber includes therein one or more structures serving to produce a larger surface for evaporating the urea water solution. This structure(s) may be a structured surface to be obtained by applying a coating to the inner surface of the evaporating chamber. Further, the evaporating chamber is connected to a hydrolysis catalytic converter via the second opening, and this hydrolysis catalytic converter is directly connected to an exhaust pipe. This hydrolysis catalytic converter has temperature controlling means comprising a heating wire wound around the hydrolysis catalytic converter itself.
The mixed gas supplying device constituted in the above manner is configured to produce a mixed gas from a urea water solution by the evaporating device, and the mixed gas contains at least urea, and already contains ammonia produced by virtue of thermal decomposition of the urea in an occasional manner. This mixed gas is introduced into the hydrolysis catalytic converter through the second opening, in a manner to conduct substantially complete hydrolysis of the urea into the ammonia at the hydrolysis catalytic converter, thereby producing a reducing agent mixed gas containing ammonia.
Meanwhile, there has been disclosed an apparatus for treating exhaust gas of an internal combustion engine, where a hydrolytic catalyst is connected to at least one feed line for feeding a water solution containing urea, an exhaust gas flows through an SCR catalyst, and a rod-shaped heating element for heating at least one of at least part of the feed line and the hydrolytic catalyst is arranged (see Patent Document 4, for example). In this exhaust gas treating apparatus, at least one of at least part of the feed line and the hydrolytic catalyst is arranged around the rod-shaped heating element. The rod-shaped heating element is surrounded by a casing tube, which is formed in one piece with the rod-shaped heating element or connected to the rod-shaped heating element by a material connection. The casing tube is provided with a channel therein. Here, the channel comprises one or more channels, each formed in a substantially spiral shape around the rod-shaped heating element and each having an annular gap cross-section internally delimited by the casing tube and externally delimited by the sleeve. The rod-shaped heating element is configured to evaporate a urea water solution at a first zone of the channel, in a manner to cause a mixed gas to flow through a second zone of the channel. Provided at the second zone of the channel is a coating for promoting hydrolysis of urea into ammonia, so that the second zone of the channel is used as a hydrolyzing channel and as a hydrolytic catalyst. After hydrolysis of urea into ammonia, a vapor stream containing ammonia is supplied as a reducing agent from the channel into an exhaust pipe. Further, the sleeve is pushed over the casing tube. This sleeve may, for example, itself have suitable heat conductors, so that the sleeve is also heatable, and the channel is thus heated from both outside and inside.